The image of an infinitely distant object will fall in front of the retina in myopia (nearsightedness), on the retina in emmetropia (normal sightedness), and behind the retina in hyperopia (farsightedness), when these eyes are exerting zero accommodation. The emmetropic eye forms sharp retinal imagers of distant objects with the lens of the eye in relaxed accommodation. This ideal optical human condition of emmetropia is possible as a result of a function of corneal curvature and axial length of the eye and takes into account that parallel rays of light travelling from air will bend when passing through the cornea surface and into the liquid environment of the eye. Normally, the emmetrope can see distant scenes sharply and, in addition, can see objects held close to the eye without awareness of any focusing by the eye. The process of focusing upon a near object, called accommodation, is accomplished by the muscles of the ciliary body of the eye contracting to vary the shape of the crystalline lens of the eye. To see at a distance, the ciliary muscles are relaxed; to see nearby, the ciliary body contracts to reshape the lens. The amount of accommodation exerted from the relaxed state of the muscles of the ciliary body to the contracted state of the ciliary muscles (i.e., to full accommodation) of the eye is termed the amplitude of accommodation. When the eye is fully accommodated, the point in space which is focused upon the retina is called the near point of the eye, or the nearest point of distinct vision.
Accommodation is measured in diopters. A diopter is defined as 1/the distance in meters to the near point of vision. In both emmetropic individuals and myopic individuals, who have been treated by corneal surgery, the ability to accommodate is gradually lost with age. In fact, the ability to reshape the lens to focus upon a near point may be completely lost after age 40 years. This decrease in the amplitude of accommodation and the consequent loss of near vision is called presbyopia and is thought to be a normal part of the aging process. The inverse relationship between age and the amplitude of accommodation can be seen in Table 1.
TABLE 1 Relationship Between Age, Amplitude of Accommodation and Near Vision for Emmetrope Near Point Amplitude of Accommodation For Emmetrope Age (Diopters) (cm) 10 14.0 7.0 20 10.0 10.0 30 7.0 14.2 40 4.5 22.2 45 3.5 28.5 50 2.5 40.0 55 1.75 57.0 60 1.00 100.0 65 0.50 200.0 70 0.25 400.00
Physiologically, accommodation is under the influence of the parasympathetic nervous system and occurs through the chemical action of acetycholine on muscle fibers of the ciliary body. Contraction of the ciliary body muscles decreases the tension of the lens ligaments, which allows the lens to focus at near point.
Acetylcholine, when working on the eye or other smooth muscles of the body is regulated by the natural cholinesterase enzyme which breaks down acetylcholine and thus turns off its parasympathetic effect on muscles. In an effort to correct presbyopia, the effect of acetylcholine on the muscles of the eye could be increased either by adding an acetylcholine like drug such as pilocarpine, or by blocking the breakdown of acetylcholine with a drug which inhibits the natural cholinesterase (e.g., a cholinesterase inhibitor).
There have been problems with the first approach: When pilocarpine hydrochloride, an acetylcholine like drug, sold as SALAGER.RTM. (MGI Pharma, Minnetonka, Minn.), is applied to an emmetropic eye, the increased parasymathetic effect leads to enhanced near vision but at the sacrifice of distant vision. The emmetropic eye becomes myopic as a consequence of this adverse side effect, thus acetylcholine treatment to correct presbyopia has not been effective. Likewise, the second approach, the use of cholinesterase inhibitors, has been unsuccessful because of similar side effects from the cholinesterase drugs used in current concentrations. No other pharmacological agents have been found to restore near vision in an individual with presbyopia. Thus presbyopia is considered untreatable with current pharmacological agents.